KIP-639 — Move nodeLevelSensor and storeLevelSensor methods from StreamsMetricsImpl to StreamsMetrics

Discussion StreamsMetrics

Promotes `nodeLevelSensor()` and `storeLevelSensor()` helper methods from the internal `StreamsMetricsImpl` class to the public `StreamsMetrics` interface so custom state stores and processors can register sensors at these granularity levels safely. These internal helpers add null-safety guards for sensor registration but are inaccessible to user-implemented stores without casting to the internal class.

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